The physical-chemical properties of lipid membranes, which are crucially important to many biological processes, are been investigated by atomic force microscopy (AFM) in this project with NIAID collaborators. Quantitative features of the main phase transition in 2-dimyristoyl-sn-glycero-phosphocholine (DMPC) have been resolved by tapping-mode AFM images in an environmentally controlled chamber at various temperatures. These AFM images reveal two membrane phases near the expected DMPC chain-melting temperature. We have quantified the marked thinning and mechanical softening of the DMPC membrane upon chain melting from precise AFM thickness measurements. We have also constructed a novel biophysical model, which permits an estimate of the thermodynamic transition enthalpy, entropy, and the membrane phase domain size from our AFM-acquired temperature-dependent phase distributions. The unique AFM technology developed at NIAID is being pushed further to elucidate membrane-associated biomolecular events critical to medically important processes.